| August 2, 1939 | Einstein Signed the Einstein-Szilard Letter | On August 2, 1939, Albert Einstein signed the Einstein-Szilard Letter. This letter encouraged Roosevelt to actively begin work on an atomic weapon to potentially take action against the growing power of Nazi Germany. |
| September 3, 1939 | World War II Began in Europe | World War II began with the Nazi invasion of Poland in 1939. Britain provided an ultimatum, and when the time ticked down on that ultimatum, war began. |
| October 11, 1939 | Einstein-Szilard Letter Delivered to Roosevelt | The Einstein-Szilard letter was delivered to Roosevelt by economist Albert Sachs. Roosevelt immediately established a Uranium Committee to begin research. |
| October 21, 1939 | First Meeting of Uranium Committee | Lyman Briggs handled the first meeting of the Uranium Committee. The Committee was given a $6,000 budget to begin their work. |
| March 2, 1940 | Neil Bohr's Hypothesis Verified | Neil Bohr's hypothesis that Uranium 235 is responsible for fission is verified at Columbia University. |
| June 12, 1940 | National Research Defense Committee Created | In June 1940, the National Research Defense Committee was created, absorbing the Uranium Research Committee. The budget was increased to $40,000. |
| February 25, 1941 | Discovery of Plutonium | Plutonium was conclusively discovered on February 25, 1941 by Glenn Seaborg and Arthur Wahl. |
| June 28, 1941 | Office of Scientific Research and Development Created | The Office of Scientific Research and Development was created on June 28, 1941. The OSRD absorbed the National Research Defense Committee. |
| December 7, 1941 | Attack on Pearl Harbor | On December 7, 1941, the Japanese attacked the U.S. naval base at Pearl Harbor, Hawaii. This forced the immediate entry of the United States into World War II. |
| January 19, 1942 | Roosevelt Authorized Atomic Bomb Project | In January 1942, Franklin D. Roosevelt authorized the Manhattan Project to develop atomic weapons to use against the enemies of the United States. |
| September 29, 1942 | Acquisition of Site X | The U.S. government acquired land in Oak Ridge, Tennessee as the first research site for the Manhattan Project. This was called Site X. |
| November 16, 1942 | Los Alamos Declared Site Y | Los Alamos, New Mexico was chosen as Site Y for the development of an atomic bomb. |
| April 1, 1943 | Los Alamos Laboratory Established | The research laboratory at Los Alamos was established in April 1943. This research laboratory would be fully responsible for the development of the atomic bomb. |
| January 11, 1944 | Special Group Appointed to Study Implosion at Los Alamos | On January 11, 1944, a special group was appointed to study the possibilities of implosion rather than explosion. |
| July 4, 1944 | Thin Man Abandoned, Fat Boy Prioritized | The explosion-based Thin Man project was fully abandoned in July 1944. Fat Boy, an implosion based weapon, and Little Boy, a gun-based weapon, remained in development. |
| May 7, 1945 | Surrender of Nazi Germany | Nazi Germany surrendered on May 7, 1945, or Victory Europe day. The war in the Pacific raged on. |
| July 16, 1945 | Test of the Gadget | On July 16, 1945, the U.S. conducted the first test of an atomic weapon, called the Gadget. |
| July 25, 1945 | Atomic Weapon Use Authorized | The United States authorized the use of atomic weapons against key targets in Japan, including Hiroshima and Nagasaki on July 25. The following day, the U.S. provided Japan with a final warning about devastating damage. |
| August 6, 1945 | Little Boy Deployed on Hiroshima | On August 6, 1945, Little Boy was deployed by the B-29 bomber Enola Gray. Little Boy was a gun-type Uranium 235 based weapon. |
| August 6, 1945 | Fat Man Deployed on Nagasaki | As the Enola Gray bombed Hiroshima, another B-29, the Bockscar, dropped a plutonium based implosion bomb on Nagasaki. |
| August 12, 1945 | Surrender of Japan | Japan offered its unconditional surrender on August 12, 1945, commonly called Victory Japan day in the United States. |
| January 1, 1947 | Atomic Energy Act of 1946 | The Manhattan Project was officially dismantled on January 1, 1947 in response to the Atomic Energy Act of 1946. |
